Seed crystals and methods using the same



Sept. 6, 1966 s. R. BHOLA 3,271,118

SEED CRYSTALS AND METHODS USING THE SAME FiledvOct. 18, 1961 INVENTOR Srb' Ram 5720M ATTORNEY United States Patent 3,271,118 SEED CRYSTALS AND METHODS USING THE SAME Siri Ram Bhola, St. Louis, Mo., assignor to Monsanto Company, a corporation of Delaware Filed Oct. 18, 1961, Ser. No. 145,866 19 Claims. (Cl. 23-301) This invention relates to improved crystalline seeds suitable for use in growing crystals from molten material. More particularly the invention relates to crystalline seeds which facilitate the production of crystals having a desirably low level of defects and to methods for using such seeds in the production of high quality crystals from a molten supply of material.

Crystals are commercially grown for a number of different applications such as for use in the manufacture of radio or telephone oscillators and in the manufacture of semiconductor devices. In producing crystals for such applications it is frequently desirable to contact a molten body of material with a seed crystal and thereafter slowly allow material to solidify from the molten supply onto the seed so that a relatively large, as compared to the size of the seed, single crystal of material results.

For most applications it is desirable that crystals have as nearly perfect a structure as is possible or at least a controlled level of defects and the difficulties of producing such crystals are well known in the art. In efforts to improve crystal quality great stress has been laid upon obtaining seeds as nearly perfect in crystalline structure as possible because it is recognized that any defect in a seed crystal is normally carried over into a larger monocrystal grown on the seed. In spite of great improvement in seed quality, however, it is by procedures presently available extremely dificult to produce from molten material crystals having a very low defect level or even crystals of a controlled defect level. In addition, the cost of seeds has become a major expense since only the highest quality seeds can be employed if a good quality product is expected and it is usually not possible to reuse seeds.

It is a primary object of this invention to provide crystalline seeds which can be employed to produce from a molten supply of material crystals with a lower defect level than it has heretofore been commercially feasible to produce under comparable conditions of crystal growth.

It is another object of the invention to provide crystalline seeds which can be employed to make monocrystals having a level of defect within a selected range.

It is a further object of the invention to provide seed crystals which can be reused to produce high quality monocrystalline material from molten material.

The above as well as other objects of the invention are accomplished by the provision of a crystalline seed comprising an elongated body of monocrystalline material so shaped that there is a sharply demarcated zone of substantially reduced cross-sectional area intermediate the ends thereof. It has been found that much of the difficulty heretofore experienced has been due to the generation in the seed crystal of defects as a result of thermal shock and the spreading of the defects thus generated throughout a major portion of the crystalline seed. Since the generation and spreading of defects caused in this manner is a largely uncontrollable phenomenon and since the defects are propagated in a monocrystal grown upon the seed, the high and unpredictable defect level in the grown crystal is thus explained, and While applicant does not wish to be limited to any particular theory, it is believed that the sharply demarcated zone of substantially reduced cross-sectional area in a crystalline seed according to this invention acts as a trap to prevent or reduce the spreading of defects. More specifically, the zone acts one side of the seed crystal.

3 ,2 7 1,1 18 Patented Sept. 6, 1966 to prevent the spread of edge dislocations generated as a result of thermal shock when the seed crystal is brought into contact with a body of molten material from which it is desired that a single crystal be grown. As a result a portion of the seed crystal is relatively undamaged and can be employed to determine the crystalline structure of a monocrystal grown from molten material With which the seed is in contact.

The invention will now be more particularly described with reference to the accompanying drawings in which:

FIG. 1 is a side elevational view of a crystalline seed in accordance with this invention.

FIG. 2 is a cross-sectional view taken substantially along the line 22 of FIG. 1.

FIG. 3 is an elevational view of a second form of seed crystal.

FIG. 4 is a cross-sectional view taken substantially along the line 44 of FIG. 3.

With particular reference to FIGS. 1 and 2 of the drawings, there is illustrated a crystalline seed, generally indicated by the reference numeral 10, having a tapered end section 12 mounted in a chuck 14 illustrated in dotted lines. Tapering of the end 12 is advantageous in some instances for the reason that it results in the seed being more rigidly held by the chuck 14. The crystalline seed is shown in this instance as being generally in the shape of an elongated rod of substantially square cross section but it can be of any other suitable shape and can, for example, be round or hexagonal in cross section. At least the untapered end portion of the seed extending from chuck 14 is formed substantially entirely from a single crystal.

Disposed downwardly from an end face 16 of seed 10 is a zone of reduced cross-sectional area generally indicated by the reference numeral 18 and illustrated in this instance as being formed by a set or pair of opposed transversely extending notches 20 and 22. Notches 20 and 22 are spaced from each other longitudinally of the seed 10 so that the zone of reduced cross section is to a degree dis-continuous but since the mean cross-sectional area is reduced in the region extending from an upper face 24 of transversely extending notch 20 to a lower face 26 of transversely extending notch 22, the entire region between faces 24 and 26 is considered as a single zone of reduced cross-sectional area for purposes of this invention.

Between notch 22 and tapered portion 12 of crystal 10 is a second set of transversely extending notches 28 and 30 which are also spaced from each other longitudinally of the rod shaped seed crystal 10. It will be seen that notches 28 and 30 form a second zone of reduced crosssectional area generally identical to that indicated by the reference numeral 18.

Each of the sets of notches illustrated in FIG. 1 of the drawings has been shown as comprising two notches and this is a preferred construction since it is the smallest number of notches that can be cut such that any line extending through the crystal 10 parallel to the longitudinal axis thereof is severed by at least one notch in each set of notches. An overlapping arrangement of this type represents the most efiicient block against the spread of dislocations that has yet been discovered. should be emphasized, however, that any substantial reduction in cross-sectional area reduces the spread throughout the seed crystal of dislocations generated by thermal shock so that the notches need not overlap and may be of any desired depth. Further, a substantial improvement can be obtained by means of a zone of reduced cross-sectional area formed by a single notch cut from It will also be apparent, in View of the above, that a set of notches may comprise a number of notches in excess of two, for example, three or four notches, which may or may not form an overlapping arrangement, but this in most instances results in the width of the zone of reduced cross-sectional area along the longitudinal axis of the seed being increased above that resulting from the use of -two notches, and for reasons which will subsequently be made apparent this is not usually desirable.

It should also be mentioned that while only two sets of notches have been illustrated in FIG. 1, any number of sets of notches may suitably be present if apparatus specifications do not limit the length of seed crystals which can be employed. However, with crystal-growing apparatus presently available it is seldom if ever advantageous to employ a seed crystal of sufficient length that more than about sets of notches can readily be inserted, and with some apparatus it is best to employ only 1 or 2 sets of notches.

With particular reference to FIG. 3 of the drawings there is illustrated a rod-shaped seed crystal 32 which is illustrated as having a generally circular cross section but which can suitably be of any desired cross-sectional configuration. The rod 32 can suitably be formed from a single crystal of any desired material and is illustrated as having one end held by a chuck 34 (shown in dotted lines) which can be of conventional construction.

Spaced from the unsupported. end face 36 of crystal 32 is a sharply demarcated zone substantially reduced cross-sectional area generally indicated by the reference numeral 38 and in this instance illustrated as comprising an annular notch circumscribing the periphery of seed 32. The notch 40 can be of any desired depth but for reasons which will subsequently be explained it is preferably as deep as is consistent with adequate structural strength of the seed 32.

Seed crystals in accordance with this invention can be prepared using conventional crystal working techniques. For example, seed crystals having a generally cylindrical configuration can be suitably formed from a melt by means of a slim rod crystal puller or the like. Alternatively, suitable seeds can be sawed, using conventional equipment, from a large single crystal of material or, less desirably, from a crystal having a large area which is monocrystalline in nature. In instances where the entire seed is not formed from a single crystal, at least the end portion which is designed to contact a melt of material should be rnonocrystalline in nature and the twinned or polycrystalline area should be on the end of the seed which is designed to be inserted into a suitable chuck. Almost any type of commercially available saw which is conventionally employed for cutting crystals can suitably be employed to form the notches as illustrated at 20 and 22 in FIG. 1 or at 40 in FIG. 3 and conventional equipment can likewise be employed for tapering one end of the seed where such tapering is necessary because of the particular chuck employed.

For best results seed crystals should normally be lapped to remove damaged areas produced by cutting or sawing and seeds prepared in accordance with this invention are no exception. The lapping operation can be performed either before or subsequently to the notching of the seeds but is preferably performed prior to notching since seeds which have been notched but not etched are quite fragile. A lapping operation, in instances where it is to be performed, can be done using conventional techniques and materials.

It is conventional practice to etch seed crystals before using the same and it is quite advantageous to effect a chemical etch of seed crystals prepared in accordance with this invention. Not only does an etching operation remove surface layers of the seed crystal which have suffered structural damage and remove contaminant elements introduced by handling and lapping, but etching also smooths or rounds abrupt surface irregularities particularly at the base of the notch or notches formed in the seed crystal in accordance with this invention. This adds appreciable strength to the notched seed crystal and lessens the danger of the seed crystal being fractured during usage. Conventional etchants can be employed in producing seed crystals in accordance with this invention. For example, in etching silicon and germanium seeds an etch comprising a mixture of nitric, hydrofluoric, and acetic acids, such as the etch known in the art as CP-4, can suitably be employed.

Seed crystals made in accordance with this invention can be formed with their longitudinal axis extending parallel to any desired axis of the crystal. In the case of silicon and germanium crystals or crystals of other materials which crystallize with a diamond cubic or zinc blend structure, seeds are normally formed or cut with their longitudinal axis parallel to the (111) or planes of the crystal since it has been found that good monocrystalline materials can most readily be grown from seeds formed in this manner.

While seeds made in accordance with this invention can be used in substantially any instance where a monocrystal is to. be grown from a molten body of material, the seeds are particularly advantageous for use with zone refining techniques for the preparation of monocrystalline semiconductor crystals. Conventional zone refining techniques for manufacturing monocrystalline semiconductor materials are well known in the art. The zone refining technique most widely employed at present comprises mounting a rod of polycrystalline semiconductor material in a vertical plane such that the rod is suspended at its upper end with its lower end unsupported, forming a molten zone at the lower extremity of the rod, contacting the molten zone with a seed crystal, slowly raising the heating means so that the molten zone traverses the length of the suspended rod from the bottom to the top, and thereafter repeating the procedure of moving a molten zone from the bottom of the rod to the top of the rod until the rod is transformed into a single crystal. Excellent quality semiconductor monocrystals, for example of silicon and germanium, are prepared commercially by this technique.

Seed crystals made in accordance with this invention can be employed in a conventional manner in a zone refining operation, and when so employed have the ad vantage that they can readily be reused since they still contain one or more areas of low dislocation density upon which a larger crystal can be grown from a molten supply of material. As previously mentioned, the dislocations generated as a result of heat shock in using a seed crystal made in accordance with this invention are largely prevented from spreading throughout the seed crystal by the sharply demarcated zone or zones of reduced cross-sectional area.

A novel procedure for employing a seed crystal in accordance. with this invention when float zone refining a crystalline material comprises initiating all zone passes except the last above the sharply demarcated zone of reduced cross-sectional area in the seed and starting the last zonal passage immediately below the zone or reduced cross-sectional area so that the crystalline structure of the finished grown crystal is determined by a portion of the seed crystal having a low dislocation density. For example, in using a seed as illustrated in FIG. 1 of the drawings, the face 16 of the seed crystal would be brought into contact with molten material formed on the lower extremity of a suspended rod to be float zone refined. This would result in a large increase in dislocation density in the material forming the portion of the seed between face 16 and the face 24 of notch 20 but would result in very little if any increase in the dislocation density of the material below the notch 22. The initial zone passages would be initiated in the portion of the seed crystal above notch 20 but the final zone passage would be started just below notch 22 so that the notches 20 and 22 would be melted out and the monocrystalline product produced from the rod being zone refined would have a crystalline structure low in dislocation density as determined by the crystalline nature of the portion of the seed above notch 28. The seed crystal could then be broken olf, for example by using a diamond scribe, and reused in the same manner with initial zone passes during the reuse of the crystal being started above notch 28 and terminal zone pass-age during reuse of the seed being started irrnnediately below notch 30.

From the above it will be seen that the first zone of reduced cross-sectional area should be spaced from the end face of the crystal intended to contact a molten supply of material a distance suflicient to avoid melting Oif the zone until it is desired that the zone be eliminated. In other words, if the notches 20 and 22 are fused together when face 16 is brought into contact with a molten material, the notches do not perform their intended purpose. On the other hand, the zone of reduced crosssectional area should not be too far removed from the end face of the crystal since this requires that the seed be unnecessarily long. Generally, the distance between the end of the crystal and the zone of reduced cross section, i.e., the distance between face 16 and face 24 of notch 20 in FIG. 1, or the distance between face 36 and notch 40 in FIG. 2 of the drawings, is preferably from about 0.2 or 0.5 to times the maximum diameter of the seed and optimum results are usually obtained when the distance is from about 1 to 2 times the maximum diameter of the seed. For similar reasons the distance between sets of notches or the like, in instances Where the seed is provided with more than one zone of reduced cross-sectional area fram the termini of the monocrystalline portion of the rod which can be effectively used for determining the structure of a growing crystal, for example, from the untapered portion of the seed crystal illustrated in FIG. 1 of the drawings, is also preferably within the broad limits specified above, with optimum results again usually being obtained when the distance is from 1 to 2 times the maximum diameter of the seed.

For a number of reasons the zone or zones of reduced cross-sectional area in seed crystals in accordance with this invention are preferably as narrow as is consistent with economical manufacturing practices and, in instances where the zone is formed by .a plurality of overlapping notches, as is consistent with adequate seed crystal strength. A primary reason why this is true is that a narrow zone of reduced cross-sectional are-a is easier to melt through than a wide zone and this is particularly true when heating is by means of an RF coil. If the zone is too wide it can result in heating on either side of the zone without the zone being entirely melted or it can result in severe necking or an unstable condition when melting through the zone. Another reason that a narrow zone is preferable is that it permits the desired results to be accomplished with a shorter seed crystal. In agreement with the above it has been found that when the zone of restricted cross-sectional area is provided by a plu- 'rality of notches, the width of each of the notches is preferably from about 0.25 to 10 times the distance the notches are spaced from each other longitudinally of the seed crystal and that the width of the zone of reduced cross-sectional are-a is from about 10% to 200% of the maximum diameter of the seed crystal. In instances where the zone of reduced cross-sectional area is provided by a single notch, the width of the zone preferably corresponds to the thickness of the most satisfactory saw blade available for forming the notch. Normally this will vary from about 0.3 to 5 mm. depending upon the material from which the seed is formed and the depth of the notch.

Having thus defined my invention and several specific embodiments thereof, what I desire to claim and secure by Letters Patent is:

1. A crystalline seed comprising an elongated crystalline body, at least one end portion of which is formed substantially entirely from a single crystal, said portion having at least one longitudinally extending, intermediate zone having a substantially reduced cross-sectional area relative to the cross-sectional area of the portions of said seed disposed immediately adjacent said one zone, said zone of reduced cross-sectional area being formed by at least one notch intermediate the termini of said end portion, the opposed faces of said notch lying in substantially parallel planes.

2. A crystalline seed of semiconductor material according to claim 1 where said zone of substantially reduced cross-sectional area is formed by an annular notch circumscribing said end portion intermediate its two termini, the opposed faces of said notch lying in substantially parallel planes.

3. A crystalline seed according to claim 1 having a plurality of notches closely spaced longitudinally of said elongated body to thereby form said zone of substantially reduced cross-sectional area, said plurality of notches collectively extending substantially across said elongated body.

4. A seed according to claim 3 where the surfaces of said notches are etched to reduce the tendency of said elongated body to fracture.

5. A seed according to claim 1 having a plurality but not more than about five zones of reduced cross-sectional area.

6. A seed comprising an elongated rod of crystalline material, at least one end portion of said rod being formed substantially entirely from a single crystal, said portion having formed therein, in a zone spaced from the end termini of said portion, at least one pair of transversely extending notches spaced from each other longitudinally of said rod, the depth and configuration of said notches being such that all lines extending through said portion parallel to the longitudinal axis thereof are severed by at least one of said notches.

7. A seed according to claim 6 having from two to five pairs of notches.

8. A seed according to claim 7 formed from elemental silicon.

9. An elongated rod of monocrystalline semiconductor material suitable for use as a seed and having from one to three sets of notches disposed longitudinally thereof, each of said sets comprising a pair of transversely extending notches spaced from each other longitudinally of said rod, the depth of said notches being such that jointly said pair of notches extend substantially across said rod, the width of said notches and the spacing of said notches longitudinally of said rod being such that the distance between the two most remote notch faces in each of sets of notches is from about 10% to 200% of the diameter of said rod.

10. A rod according to claim 9 wherein the width of each of said notches is from 0.25 to 10 times the distance the notches in each of said sets are spaced from each other longitudinally of said rod.

11. A rod according to claim 10 in which the depths and configurations of the notches in each of said sets is such that any line extending longitudinally through said rod is severed by at least one of said notches in each of said sets.

12. A rod according to claim 9 having a plurality of sets of notches, each of said sets being spaced from any other set and from the ends of said rod a distance at least equal to the maximum diameter of said rod.

13. A rod according to claim 12 where the surfaces of said notches are etched to reduce the tendency of said rod to fracture.

14. A rod according to claim 13 formed of elemental silicon.

15. A rod according to claim 13 formed from germanium.

16. A rod according to claim 11 having a plurality of sets of notches, each of said sets being spaced from any other set and from the end of said rod a distance at least equal to the maximum diameter of said rod.

17. In a multipass method of zone refining a cylinder of polycrystalline material to transform at least a major portion thereof into a single crystal and wherein a crosssectional molten zone is passed unidirectionally through at least a longitudinal portion of said cylinder for a plurality of zone passages, the improvement which comprises cutting a notch in at least one side of a rod-shaped monocrystalline seed crystal to form a sharply demarcated zone of substantially reduced cross-sectional area in the seed spaced from the ends thereof, contacting a molten end segment of said cylinder with one end of said rodshaped monocrystalline seed spaced from said sharply demarcated zone of substantially reduced cross-sectional area having a relatively high defect density such that said area, the spread by thermal shock of defects through said seed crystal being limited by said zone so that the portion of said seed on the side of said zone remote from said end which is brought into contact with said molten segment of said cylinder has a low defect density relative to the portion of said seed which is thermally shocked by being brought into contact with said molten segment of said cylinder, initiating at least one zone passage in the portion of said seed above the sharply demarcated zone of reduced cross-sectional area in the seed and advancing a molten zone upwardly into said cylinder from the portion of said seed above said sharply demarcated zone of substantially reduced cross-sectional area having a relatively high defect density such that said zone of reduced cross-sectional area remains unmelted, and thereafter starting the last zone passage immediately below the zone of reduced cross-sectional area in the seed and melting in said last zone passage through said zone of reduced crosssectional area, so that the crystalline structure of the finished zone refined crystal is determined by a portion of the seed crystal having a low defect density.

18. A method according to claim 17 wherein said molten end segment is contacted with said seed at the initiation of the first zone passage.

19. A method according to claim 17 wherein said cylinder and said seed in each instance are formed of elemental silicon.

References Cited by the Examiner UNITED STATES PATENTS 2,517,661 8/1950 Hart. 2,927,008 3/ 1960 Shockley 23-307 X 3,177,051 4/1965 Scholte 23-301 X orHER REFERENCES NORMAN YUDKOFF, Primary Examiner.

GEORGE D. MITCHELL, Examiner.

G. P. HINES, A. J. ADAMCIK, Assistant Examiners. 

1. A CRYSTALLINE SEED COMPRISING AN ELONGATED CRYSTALLINE BODY, AT LEAST ONE END PORTION OF WHICH IS FORMED SUBSTANTIALLY ENTIRELY FROM A SINGLE CRYSTAL, SAID PORTION HAVING AT LEAST ONE LONGITUDINALLY EXTENDING, INTERMEDIATE ZONE HAVING A SUBSTANTIALLY REDUCED CROSS-SECTIONAL AREA RELATIVE TO THE CROSS-SECTIONAL AREA OF THE PORTIONS OF SAID SEED DISPOSED IMMEDIATELY ADJACENT SAID ONE ZONE, SAID ZONE OF REDUCED CROSS-SECTIONAL AREA BEING FORMED BY AT LEAST ONE NOTCH INTERMEDIATE THE TERMINI OF SAID END PORTION, THE OPPOSED FACES OF SAID NOTCH LYING IN SUBSTANTIALLY PARALLEL PLANES. 